Fuel cells using solid polymer electrolyte (SPE) membranes are expected to find widespread use as power supplies for electric cars and small-size auxiliary power supplies due to a low operating temperature below 100° C. and a high energy density. In such SPE fuel cells, constituent technologies relating to electrolyte membranes, platinum base catalysts, gas diffusion electrodes, and electrolyte membrane/electrode assemblies are important. Among others, the technologies relating to electrolyte membranes and electrolyte membrane/electrode assemblies are most important because they largely govern the performance of fuel cells.
In SPE fuel cells, an electrolyte membrane on its opposite sides is combined with a fuel diffusion electrode and an air diffusion electrode so that the electrolyte membrane and the electrodes form a substantially integral structure. Then the electrolyte membrane not only acts as an electrolyte for conducting protons, but also plays the role of a diaphragm for preventing a fuel (such as hydrogen or methanol) from directly mixing with an oxidant (such as air or oxygen) even under applied pressure.
From the electrolyte aspect, the electrolyte membrane is required to have a high proton transfer velocity, a high ion exchange capacity, and a high and constant water-retaining ability enough to maintain a low electric resistance. The role of a diaphragm requires the electrolyte membrane to have a high dynamic strength, dimensional stability, chemical stability during long-term service, and no extra permeation of hydrogen gas or methanol as the fuel and oxygen gas as the oxidant.
Electrolyte membranes used in early SPE fuel cells were ion exchange membranes of hydrocarbon resins obtained through copolymerization of styrene with divinyl benzene. These electrolyte membranes, however, lacked practical usefulness due to very low durability. Thereafter, perfluorosulfonic acid/PTFE copolymer membranes developed by E.I. duPont and commercially available under the trade mark “Nafion” have been widely used instead.
One problem associated with conventional fluororesin base electrolyte membranes as typified by Nafion is an increased cost because their manufacture starts from the synthesis of monomers and requires a number of steps. This becomes a substantial bar against practical applications. With respect to the thickness of electrolyte membranes, as the membrane becomes thinner, proton conduction becomes easier and hence, fuel cells provide better power generation characteristics. Thin electrolyte membranes, however, can be ruptured when an electrolyte membrane and an electrode are pressed together at elevated temperature to enhance the bond therebetween.
Efforts have been made to develop inexpensive electrolyte membranes that can replace the Nafion and similar membranes. A number of electrolyte membranes under study are described in Viral Mehta, Journal of Power Sources, 114 (2003), pp. 32-53.
However, none of these electrolyte membranes have better characteristics of proton conduction, elongation and strength than Nafion. No electrolyte membrane having better characteristics than Nafion is available up to the present.